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Journal of Neuroscience, Vol 13, 181-194, Copyright © 1993 by Society for Neuroscience
Multiple modes of N-type calcium channel activity distinguished by differences in gating kinetics
AH Delcour, D Lipscombe and RW Tsien
Department of Molecular and Cellular Physiology, Beckman Center, Stanford University Medical Center, California 94305.
In many neurons, N-type Ca2+ channels are a major Ca2+ entry pathway and
strongly influence neurotransmitter release. We carried out cell- attached
patch recordings (110 mM Ba2+ as charge carrier) to characterize the rapid
opening and closing kinetics of N-type Ca2+ channel gating in frog
sympathetic neurons. Single channels display at least three distinct
patterns of gating, characterized as low-, medium- , and high-rho o modes
on the basis of channel open probability (rho o) during depolarizing pulses
to -10 mV. Spontaneous transitions from one mode to another are infrequent,
with an exponential distribution of dwell times and mean sojourns of
approximately 10 sec in each mode. Thus, a channel typically undergoes
hundreds or thousands of open- closed transitions in one mode before
switching to a different mode. Transitions between modes during a
depolarization were occasionally detected, but were rare, as expected for
infrequent modal switching. Within each mode, the activation kinetics were
well described by a simple scheme (C2-C1-O), as previously reported for
other types of Ca2+ channels. Rate constants are strikingly different from
one mode to another, giving each mode its own characteristic kinetic
signature. The gating behavior at -10 mV ranges from brief openings
(approximately 0.3 msec) and long closures (10-20 msec) for low-rho o
gating to long openings (3 msec) and brief closures (approximately 1 msec)
for high- rho o gating. Intermediate values for mean open durations
(approximately 1.5 msec) and mean closed durations (approximately 3 msec)
were found for medium-rho o gating. In addition to being kinetically
distinct, channel openings in the low-rho o mode often exhibit a unitary
current approximately 0.2 pA larger than in the medium- or high-rho o mode.
Each mode is characterized by its own voltage dependence: activation occurs
at relatively negative potentials and is most steeply voltage dependent in
the high-rho o mode, while activation requires very strong depolarizations
and is weakly voltage dependent in the low-rho o mode. The proportion of
time spent in the individual modes varies greatly from one patch to
another, suggesting that modal gating may be subject to cellular control.
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